A conventional motor vehicle alternator consists of a single-phase or multi-phase generator which generally includes a stator within which a rotor is arranged to rotate, the rotor having an excitation winding. The excitation winding is supplied with current through brushes which make contact with two slip rings arranged on a projecting portion of the rotor shaft.
It is already known, in particular from European patent specification No. EP 0 707 374A, to provide such rotary machines in which, with a view in particular to increasing their power output, the field of excitation of the rotor is produced simultaneously by permanent magnets and by excitation windings. This is generally referred to as mixed excitation. In such a machine, the current delivered by the armature of the machine (using switching means associated with the excitation windings) is controlled. Such switching means are arranged to enable the direction of excitation to be reversed selectively, thus reducing the magnetic flux of the permanent magnets, or even canceling it out.
This necessity for reversing the direction of the excitation current makes it necessary to employ a semiconductor switching bridge of the H type, which is relatively expensive and which therefore raises the selling price of the machine.